For many industrial control applications, it is necessary to send the output signal from a transducer over a long wire, and similarly, the power to operate the transducer must be transmitted over a substantial distance. When transducers, such as pressure transducers, have a voltage output, it is a common experience to see a significant amount of loss or reduction in the voltage at the other end of the long wire, due to the wire resistance. Accordingly, for many applications, it is preferred to transmit current signals rather than voltage signals, in responses to changes in pressure or other input to a transducer.
Concerning another factor which comes into play, it is desirable that the minimum current transmitted over the long wire, for example at the lowest pressure to be measured, be relatively low. In this regard, a current range of from 4 milliamperes to 20 milliamperes has become an industrial standard. Where a certain amount of current is required to power the basic voltage-type pressure transducer, it is therefore desirable to minimize the additional current required to convert from voltage output to a current output.
Accordingly, a principal object of the present invention is to provide a simple and inexpensive system for converting a voltage type transducer into a system wherein the output current is proportional to the normal voltage output variations; and wherein the power drain required for the additional conversion circuitry is minimal. Collateral objects of the present invention are to provide convenient and simple gain and offset adjustments for the output current, for compatibility with the cusomers' requirements.